<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        window.addEventListener('load',function(){
            var btn = document.querySelector('button');
            btn.addEventListener('click',function(){
                alert('哈哈哈');
            })
        })
        window.addEventListener('load',function(){
            alert('111');
        })
        window.addEventListener('DOMContentLoaded',function(){
            alert('2222');//只加载dom
        })
    </script>
    <button>开启</button>
</body>
</html>